Means for reducing lightning interference in carrier telegraph circuits



June 21, 1932.

V. P. THORP MEANS FOR REDUGI 1,863,674 NG LIGHTNING INTERFERENCE IN CARRIER TELEGRAPH CIRCUITS Filed June 19, 1930 'IIHII 'lll-lll INVENTOR WMM/l0 BY ATTORNEY June 2l, 1932. v. P. THORP MEANS FOR REDUCING LIGHTNING INTERFERENCE IN CARRIER TELEGRAPH CIRCUITS Filed June 19 2 Sheets-Sheet 2 ffy* INVENTOR 7]? Tha/7x7 BY fly@ Zw ATTORN EY Patented June 21, 1932 UNITED srATEs PArEN VAUGHN r. THOR?, or RIVER EDGE, NEW JERSEY, AssIeNoR vTo AMERicAN TELEPHONE AND TELEGRAPR COMPANY, A coaroaarioN or NEw'YoRK MEANS FOR REDUCING LIGHTNING INTERFERENCEFIN CARRIER TELEGRAIH CIRCUITS Application filed V.Tune 19,

This invention relates to means for reducmg lightning interference upon carrier telegraph circuits, and particularly to an arrangement for preventing the shock excita# @tion by lightning impulses of the tuned circuits in the receiving branches of the carrier telegraph system.

I In the operation of carrier telegraph circuits interference from lightning impulses 1Q produces distortionof the received signals, and various means have been devised to eliminate or reduce the effects of such interference. It has been found from experience l that the impulses produced by lightning in ll a circuit, such as a carrier telegraph circuit, are of short duration but of largeamplitude. When those impulses enter the receiving circuits and shock-excite the tuned circuits therein, the e'ect of such excitation is to prolong the lightning impulse sufficiently to cause the operation of the receiving relay connected with the receiving circuit. vTere it not for the prolongation of the impulse produced by the tuned circuits, the'relays would I seldom be affected by the lightning impulse,

because the normal duration ofthat impulse is less than the time required to build up the required operating current of the receiving relay. l

My invention resides in a method and means for preventing the shock excitation by lightning'impulses of the'tuned circuits in the receiving branches of a carrier telegraph system. Y

This inventionV will be clearly understood from the following description when read in connection with the attached drawings of which Figure l shows schematically the application of the invention to a carrier telegraph system; Fig. 2 shows the use of a specific type of current-limiting device, namely, a copper oxide rectifier to prevent shock excitation of the tuned circuits, and Fig. 2A is acharacteristic curve illustrating the description of the principle underlying theinvention. Y

YIn Fig. l, Lllrepresents the transmission circuit over which carrier telegraph signals maybe transmitted in `both directionsv between stations each equipped with apparatus 1930. serial No.V 462,380'.

as represented by the terminal circuit of the drawings. The line L1 has connected therewith the sending bus L2', and the receiving bus Connected with the sending bus is a plurality of branchcircuits designated'as chan nels Nos. l, 2 and 3, each having associated therewith a tuned circuitby means of which `currents of the various frequencies of transmitted signals are impressed upon the sending bus L2'. The sending branches may also have associated therewith transmitting ampliiers and transmitting apparatus of the type customarily employed for carrier telegraph transmission. Since the invention herein involved relates to the receiving circuits, it will be unnecessary to describe in detail the transmitting circuits. n

Thereceiving bus L3 has connected therewith a plurality of branch circuits 114,115 and L6, each intended and capable of receiving the signal transmitted by a carrier'frequency over the line L1 from a distant station. The branch circuit L4 contains a current-limiting devi-ce 1, that may be a copper oxide rectifier as shown in Fig. 2, or any of a plurality'of devices capable of limiting the magnitude of the voltage that they will transmit to theA series resonant circuit 2. That resonant circuit is coupled with aparallel resonant circuit 3 that is connected with the input of the amplifier-detector 4, the output of which is connected with one ofthe windings 5 of the receivingrelay 6. 'lhat relay which also has a biasing winding 7 controls the transmission of impulses over the loop circuit L7 to the subscribers circuit represented schematically by the relay 8. yL5 and L6 represent other branch receiving circuits similar, preferably,

to L4, for the reception of other carrier telegraph channels.

The current-limiting devices represented schematically by 1, 11 and 21, may be of any type whose impedance is relatively small for the normal operating voltages of the system,

"but which increases rapidly with an increase CFI lthe same numerals have been employed to designate parts similar to those shown in Fig. 1. In Fig. 2 the apparatus enclosed in the vdotted block is the current-limiting. device 1 by means of which the shock excitation of the tuned circuits 2 and 3 is prevented or substantially reduced in frequencyof occurrence. The elements of the current-limiting device shown in Fig. 2 are copper oxide rectiliers of the type fully described in the paper by Grondahl and Geiger entitled, A new electronic rectifier published in the Journal of the A. I. E. EMarch 1927 -(pages 215.- 222). The current-limiting device as vshown in Fig. 2 comprises two copper oxide elements l1a and 1b, each of which has the characteristic shown in Fig. 2-A. The arrowheads of the elements 1a and 1b indicate the low resistance direction of flow of current through them. As shown in Fig. 2 the resistance of each of the elements la and 1b is the same in each direction for zero applied voltage', and very nearly the same in each direction for voltages of small magnitudes, such, for example, as the incoming carrier voltage that is of the order of one-tenth volt. Under that condition very little rectication will occur in the elements 1 and 1b. The principal eifect of those elements at the low operating voltages is to introduce a constant resistanceof about Y 400 ohms in series with each receiving branch. Accordingly, the voltages of the carrier telegraph impulses willl cause current to flow through the tuned circuit 2 which in turn will produce oscillations in circuit 3 that will be amplified and detected by 4 `and impressed upon the receiving relay 6 to cause its operation. Y Y v Y` If, however, a high voltage such as that produced by lightning, should be impressed by the line Ll uponvthe receiving bus L3, and in turn upon the branches L4, L5V and LG, the eii'ect of the devices 1, 11 and 21 in those branches will be to prevent an abnormal current flowing in the series resonant circuits. It will be seen from Fig. 2,-A that the resistance of one 'of the copper oxide elements will increase greatly in one direction of applied voltage, and Willdecrease in the other direction, Vwhich decrease however, is of comparatively small magnitude;V For eX- ample, assume the instant when the phase of parent that while current will readily low lthrough the element 1, the resistance introduced by the element 1b will be so great for a relatively small increase of negative voltage that the current flowing through the series resonant circuit 2 will be kept down to a relatively low value by the joint action of the elements. Consequently, the oscillations set up in the tuned circuit '3. will not cause the operation of the receiving relay 6. With the 1 phase-reversalof the interfering voltage wave in the branch L4 the element 1a will similarly serve to introduce a large resistancein series with lthe tuned circuit. Y

A. device Vof the copper oxide type, as ldescribed in the Grondahl'and Geiger articleL above referred' to, has been shown as cur- .85

rent-limiting device, but it will be apparent that other forms of such devices, either of the contact type, orv the thermionic type, such as a vacuum tube, ,may be employed to limit the magnitude of the current that will flow through the series resonant circuit 2 as the result of the voltage'impressed thereon by the input circuit. Therimportant thing is to preventthe shock excitationl of the tuned circuits that wouldtend to effectively lengthenthe'fff impulse impressedl upon the receiving branches of the carrier telegraph circuit, which impulse, as in the case of a lightning surge, would not, in the absence of such prolongation, be of suilicient duration to build up a current of suiiicient magnitude to operate thereceiving relay. s

While .the invention has been disclosed schematically, it is to be understood that it Vis capable of embodiment informs other than those shown, without departing from the spirit and scope of the appended claims.

What is claimed is s.I

1. In a carrier telegraph system, the comd bination with a transmission circuit exposed to lightning interference, of a receiving circuit connectedv with the said v transmission circuit and containing a tuned circuit, and means connected in series with each side of saidV tuned circuit to automatically dampen the tuning thereof, each of said damping means oifering low resistance to current of one polarity and high resistance to the other polarity, the said damping means being connected effectively in opposition.

2. In a carrier telegraph system the combination with a transmission circuit exposed to static interference, of a circuit tuned to the frequency of a carrier channel, a signal responsive 'device intended to be actuated by voltage impulses of the carrier frequency` effectively connected to the said tuned circuit, and means connected in Vserieswith each side of said tuned circuit to automatically dampen the tuning thereof, yeach of said damping June 1930.

VAUGHN P. THORP 

